The present invention relates to data communications, and more particularly to differential demodulators, especially for phase shift keyed (PSK) signals.
Phase shift keying or digital phase modulation as it is frequently called is widely used by the data communications industry. In PSK data communication systems digital information is conveyed in the form of phase differences between successive symbol periods of a carrier wave. The receiver derives the digital information by detecting the changes of phase of the incoming signal. For example, a detectable change in phase can be arbitrarily assigned a digital value of "1", whereas no detectable change in phase can be assigned a digital value of "0". More detailed descriptions of PSK communication techniques are found in Lucky, Salz, and Weldon, Principles of Data Communication, Chapters 3 and 9 (McGraw Hill 1968) and in R. W. Bennett and J. R. Davey, Data Transmission, Chapter 10, (McGraw Hill 1965).
Two general methods of digital phase detection (PSK demodulation) are coherent detection and differential detection. Coherent detection employs a phase reference and a carrier tracking loop at the receiver. This method, theoretically, is the optimum for detection of PSK signals. However, performance of such a demodulation scheme is severely degraded when the signals are subject to secondary phase-shifting and noise due to less than optimum propagation conditions.
Differential detection (also known as differentially coherent detection) utilizes a phase reference derived from past signaling intervals to demodulate a PSK signal. In this technique an incoming symbol (a binary digit or bit) is delayed by a time delay network for one symbol period (T.sub.b) to act as a phase reference for the next succeeding symbol. Differential detection is an attractive technique because no carrier tracking loop or internal phase reference is required. However, one problem is that, as noted by Bennett and Davey at page 208, "The delay network type of reference has the disadvantage of being somewhat more difficult to adapt to different symbol rates than other methods."
A second problem of differential detection is that at every symbol rate, the symbol delay must be very accurate to avoid significant performance degradation. Such a problem becomes serious, for example, when a received modulated signal is recorded on tape and later demodulated. Such problems as recorder jitter (i.e. variation in tape speed) introduce variations in the recording or playback speed, and corresponding variations in the symbol rate, which cause degradation in performance of the differential demodulator.